It is the objective of the proposed research to investigate the recently discovered phenomenon of photoreactivation of ionizing radiation-induced cellular damage. A careful comparison will be made between the photoreactivation of ionizing radiation-induced damage and that induced by UV light. A concerted effort will be made to ascertain 1) the mechanism of production of photoreactivatable damage, 2) the nature of the photoreactivatable damage, and 3) the mechanism of the photoreactivation process. The scientific methods to be employed will be to measure 1) cellular survival and 2) DNA damage as a function of radiation dose (either UV or X-ray) with and without exposure to photoreactivating light (lambda = 320-420 nm). The influence of primary radical (eaq;OH;H) scavengers and free radical repair agents (e.g. -SH compounds) as well as the influence of X-ray energy (and th possible involvement of Cerenkov radiation) on the yield of photoreactivatable damage will be studied. Promotion or inhibition of the photoreactivation process by chemical and physical means will be examined. This work will have significance in the further understanding of the reapir of ionizing radiation-induced damage.